1. Field of the Invention
The present invention relates generally to control apparatus and methods for generating with an inverter an outgoing signal of variable magnitude and frequency and, more particularly, to an apparatus and method which in one aspect causes the inverter to be cycled to and stopped at a preselected gating sequence when an idle condition occurs, and which in another aspect causes the gating pulse signals effectively not to be provided to the inverter from when the idle condition occurs until a designated one of the gating pulse signals is furnished.
2. Prior Art
Methods and apparatus are well known for generating an outgoing signal of variable magnitude and frequency in accordance with control signals. One such well-known approach is the use of an inverter that converts an incoming DC signal to an outgoing signal of variable magnitude and frequency in accordance with control signals. In one of the more common three-phase versions of the inverter, variable direct current (DC) power is applied to a six conduction controlled rectifying device bridge having a pair of series conduction controlled rectifying devices, such as thyristors, associated with each leg of the three-phase outgoing signal. Control signals, such as gating pulse signals, are effectively supplied to the gating terminals, or gates, of the conduction controlled rectifying devices in the inverter. The gating pulse signals cause respective ones of the conduction controlled rectifying devices to which they are effectively supplied to go to the conduction state, causing the outgoing signal of desired magnitude and frequency to be generated.
It is often desirable to cause the inverter to generate an outgoing signal in accordance with certain of the conduction controlled rectifying devices being in the conduction or gated state when, for example, the frequency of the outgoing signal is below a preselected level indicative of a low load operation. In the case of an inverter used to generate an outgoing signal provided to drive an AC electric motor, it often is important to be able to provide the outgoing signal to particular stator windings when the frequency of the outgoing signal is below a preselected value, for example, substantially zero Hertz, so that the orientation and magnitude of the flux produced by the DC outgoing signal is known.
Conventional inverters and their associated control circuitries, however, do not assure that certain thyristors are in the conduction state when the frequency of the outgoing signal is below the preselected value (indicative of low load operation) because the inverter is randomly stopped at any position of thyristors in the conduction or gated state. When the preselected frequency value is substantially zero Hertz indicative of an idle state, it becomes unlikely for the system ever to reach the desired gated thyristor pattern in the inverter if this desired gated state of thyristors is stopped at random because the outgoing signal is substantially DC and, therefore, no further gating of the inverter thyristors takes place.
It is also desirable to cause the inverter not to receive gating signals when the frequency of the outgoing signal goes below the preselected value (indicative of low operation load) until a designated one or group of the gating signals is furnished by the inverter control circuitry. By blanking the gating signals, the inverter is prevented from cycling through its conduction or gated states, and consequently providing an outgoing signal at relatively high frequency, when the conduction state of the thyristors of the inverter is shifted from the conduction state at which the inverter was randomly stopped until the designated one of the gating signals is furnished by the control circuitry of the inverter. Conventional inverters do not provide this capability of blanking the gating pulse signals from when the inverter is forced into the idle state until the designated pattern of gating signals is furnished by the control circuitry associated therewith.